This invention relates to thin film solar cells and more specifically to improvements in substrates for such cells which allows the use of low cost glass substrates in combination with high conductivity transparent conductors.
Considerable research and development has been directed towards development of commercially practical thin film solar cells. In such cells the semiconductor material, typically silicon, is on the order of 0.5 micron thick as compared to conventional single crystal silicon wafers having a thickness on the order of 250 microns. Thus a considerable savings in semiconductor material is achieved by use of thin film structures. Further savings are anticipated because thin film processes are usually more easily automated so that the labor costs per watt of output power should be considerably less than that required for the conventional cells.
An example of thin film solar cell structures is illustrated in U.S. Pat. No. 4,292,092 issued to Hanak on Sept. 29, 1981. The Hanak structure employs a glass substrate, a transparent front conductor deposited and patterned on the glass substrate, a thin semiconductor film and a metallic back contact. The performance of such cells is quite dependent upon the sheet resistivity of the transparent conductor which forms the front contact for the active regions of the solar cell. One of the primary teachings of Hanak is a laser scribing technique used to break the sheet of semiconductor material into a series of long narrow strips which are series connected to add the voltages from the individual cells while limiting the maximum current which must be carried by the various conductors. However, scribing of the material into the individual cells results in loss of some of the active area which loss must be balanced against the power savings achieved by reduction of current levels. As the sheet resistivity of the transparent conductor is reduced, the width of the individual cells can be increased, thereby reducing inactive area resulting from scribed lines as well as reducing labor and other costs of manufacture.
U.S. Pat. No. 3,677,814 issued to Gillery on July 18, 1972 teaches a method of applying tin oxide layers to glass substrates, which methods are generally applicable to thin film solar cell processes. Gillery provides examples involving the use of soda lime glass as a substrate for formation of tin oxide transparent conductors. However Gillery also teaches that lower film resistivity can be achieved if other types of glass which can be processed at high temperatures are employed.
U.S. Pat. No. 4,146,657 issued to Gordon on Mar. 27, 1979 teaches other improvements in formation of tin oxide layers on glass, which improvements are directed towards considerable reduction in sheet resistivity of the tin oxide layers. Most of the examples provided by Gordon involve the use of Pyrex glass plate as a substrate. In example 2 Gordon notes that when a "sodium free silicon substrate" is used the sheet resistivity is about one-half that achieved with a sodium bearing substrate.
The cost of the glass substrate is a major component of the total cost of the finished product. While it is desirable to achieve the lowest sheet resistivity possible in the transparent conductive layer, the cost of high temperature glass, such as Pyrex, cannot be economically justified. While lost cost and freely available soda lime glass has proven to have quite acceptable mechanical characteristics from its use in conventional solar modules, its high sodium content increases transparent conductor sheet resistivity with the adverse results discussed above.
Thus it is seen that it is desirable to provide an improved low cost soda lime glass useful in thin film solar cells or to provide a method for treating such glass substrates to prevent the adverse effects sodium ions are known to have on resistivity of tin oxide layers.